1. Field of the Invention
The present invention relates to a thermoplastic resin composition which possesses excellent stiffness properties and high heat resistance, and exhibits only small anisotropic properties during the molding operation, and superior moldability. The present invention also relates to a molded article made of the thermoplastic resin composition and to sealing method of electronic parts or photo-semiconductors using this thermoplastic resin composition.
2. Description of the Background Art
Because polyarylenesulfide resin is a thermoplastic resin which is excellent in the heat resistance, chemical resistance, electric properties, mechanical properties, dimensional resistance, and the like, it is used as a material for manufacturing precision parts and as a sealing material for electric or electronic parts.
This resin is, however, a highly crystalline polymer and exhibits conspicuously high fluidity in the temperature range above the melting point. Controlling the high fluidity during injection molding operations is difficult, resulting in production of flashes in the products and also in the significant reduction in the yield of precision parts products.
Furthermore, the high crystalline properties of the polyarylenesulfide resin can be the cause of extreme anisotropy in the characteristics of the molded products, such as molding contraction, tenacity, and the coefficient of linear expansion, along the direction of the resin flow and in the direction vertical to the flow of the resin. These are the causes of poor dimensional precision of the resin.
In order to obtain molded products with a designed dimension from this type of resin, it is essential to design molds and establish molding conditions taking the anisotropic properties of the resin into consideration. This requires special techniques, involving costs which otherwise need not be expensed.
In order to overcome this problem and to provide a resin with improved moldability, a resin composition comprising a polyarylenesulfide resin and a polystyrene resin has been proposed (Japanese Patent Publication (kokoku) No. 13469/1978). This resin composition has an extremely low heat deformation temperature and produces molded articles with impaired heat resistance. For instance, when directed to applications requiring soldering, such molded articles may be deformed by the heat of the soldering operation.
On the other hand, when a polyarylenesulfide resin composition is used as a sealing material, it exhibits only insufficient adhesion to lead frames due to the crystalline properties of the resin and depending on the other components which make up the composition. Water may invade into the interface between the sealing resin composition and the lead frame, giving rise to lowered electrical insulation and to corrosion of electrodes.
The polyarylenesulfide resin inherently exhibits low adhesion properties to metals and various organic or inorganic materials due to the characteristics of the chemical structure of this resin. For example, when an article molded from this resin is used after having been adhered to various other materials, the adhered parts may easily be released depending on the environmental conditions of use. In some cases, the article cannot be practically used. When the molded resin is used for holding parts made of other material, it exhibits only insufficient holding capability. Further, it is difficult to print the molded article or to provide ornament.
An object of the present invention is therefore to provide a thermoplastic resin composition which is free from all these drawbacks in the polyarylenesulfide resin, that is, a resin composition which possesses excellent moldability, decreased anisotropic properties, and improved adhesion to lead frames and other materials.
When the thermoplastic resin composition is used as a sealing material, it is essential that the resin have heat resistance sufficient to resist the heat at soldering temperatures. The present inventors have found that such a thermoplastic resin composition must have a Vicut softening point of 200.degree. C. or higher.
Owing to the carbon-carbon cyclic structure the norbornene-type resin used in the present invention has characteristics of high heat resistance and low moisture absorption. Because of this, the high heat resistance and low moisture absorption properties which are also characteristic to the polyarylenesulfide resin are not unduly impaired by blending a norbornene-type resin. Furthermore, because the norbornene-type resin is non-crystalline due to its bulky structure, the fluidity in the temperature range above the melting point of the crystalline polyarylenesulfide resin can be controlled by blending it with the norbornene-type resin. This can greatly improve the moldability of the polyarylenesulfide resin.
Moreover, the non-crystalline characteristic of the norbornene-type resin can greatly reduce the anisotropic properties in the molding of the polyarylenesulfide resin composition. Blending a norbornene-type resin with the polyarylenesulfide resin can also improve its adhesion properties to lead frames and various other materials.